The present invention relates to a method for forming a protective surface film on an aluminum shaped article by a non-electrolytic chemical means. The protective surface film formed according to the inventive method is very durable with good adhesion to the aluminum base and also imparted with excellent light resistance. The method is particularly suitable for the treatment of aluminum-made elements in slide fastener chains.
As is well known, shaped articles of metallic aluminum or an aluminum-based alloy (hereinafter referred to merely as aluminum) are rarely used as shaped but used after being provided with a surface film with objects not only to enhance the mechanical and chemical resistance but also to improve beautifulness. Such a protecting surface film on aluminum shaped articles is obtained either by an electrolytic method of anodization or by a non-electrolytic method of chemical conversion by use of certain chemicals. These methods have their own advantages and disadvantages. For example, an aluminum shaped article provided with a protective surface film by the anodization is very excellently resistant against corrosion so that surface treatment of most of aluminum-made building materials such as wall panels and window sashes is performed by this electrolytic method. A disadvantage of the anodization method is, on the other hand, the relatively poor fastness of color when the surface-anodized aluminum article is subjected to coloring to increase beautifulness in addition to the high costs required for the expensive electrolytic facilities.
The method of chemical conversion coating is preferred because, despite the outstanding simplicity and low costs of the treatment process owing to the absence of the complicated electrolytic treatment in the expensive facilities, the surface film obtained by the method is sufficiently corrosion-resistant and remarkably beautiful although the anti-abrasion resistance of the surface film formed by the method is somewhat inferior to that obtained by the anodization. When the aluminum article is finished by providing an overcoating with a coating composition, e.g. paints and varnishes, poor anti-abrasion resistance is less significant so that the chemical conversion method is widely practiced as a method of pre-treatment for the overcoating.
Turning now to the slide fastener chain having a plurality of elements fastened to a tape material along a periphery thereof at a regular interval, the elements are made either of a metal or a plastic synthetic resin. As a metal for shaping the fastener elements, brass, red brass and aluminum are the usually used materials. The former two materials are advantageously excellent in the anti-abrasion resistance while disadvantageous in the expensiveness on the other hand. The fastener elements made of aluminum or an aluminum-based alloy are relatively inexpensive but certain problems must be overcome when coloring of the elements is desired.
The coloring of fastener elements is conventionally undertaken either by coating with a paint or by the anodization treatment followed by dyeing of the anodized surface film in a dye solution.
The coloring in the former method is obtained by the mere adhesion of the colored coating layer to the aluminum base so that the largest problem in this method is the poor durability of the coating layer which is susceptible to wearing off or exfoliation by the repeated contact of the elements with the slider of the fastener chain or by the repeated laundering of the clothes and the like to which the slide fastener is attached.
The latter method of the anodization and dyeing is preferable in respect of the durability or anti-abrasion resistance of the colored surface when the high costs for the electrolytic treatment can be disregarded caused by the large investment for the electrolytic facilities. The most serious problem, on the other hand, is the poorly uniform electrolytic conditions for each of the fastener elements fastened in a fastener tape. This problem is unavoidable as a natural consequence of the electrolytic treatment of such a large number of fastener elements since the effects of anodization and dyeing can never be uniform unless the uniformity is ensured in the electrolytic conditions for each of the elements.
In the prior art, the uniformity in the electrolytic conditions for the elements is obtained only by providing a lead wire interwoven to the fastener tape along the peripheral portion thereof to which the elements are fastened and contacting the elements with the lead wire. Such a lead wire interwoven to the fastener tape naturally results in decreased flexibility of the slide fastener chain so that the clothers and the like with such a slide fastener chain attached thereto sometimes exhibit awkward appearance with waving or bulging in addition to the decreased smoothness in the running of the slider along the row of the elements.
Another problem in the method of anodization and dyeing of the fastener elements fastened to a fastener tape is that the tape is also dyed in the dyeing solution to cause undesired coloring of the tape simultaneously with the elements. The thus dyed tape cannot resume its original color even by careful washing, especially, when the tape is of white or light color so that the applicability of the method is limited to the fastener chains with deep-colored tapes.
A further problem in common to both of the former and the latter methods is the poor light resistance of the colored surface of the fastener elements resulting in less pleasant appearance with faded color after a relatively short period of use.